An important factor in the fabrication of infrared detectors, such as solid state photomultipliers (SSPMs) and blocked impurity band (BIB) devices, is the strict control of acceptor and donor concentrations in the doped epitaxial layers of the detectors. In fact, the reduction and control of compensating acceptors in arsenic doped silicon (Si:As) impurity band conduction (IBC) devices, for example, has been identified as a key factor in improving detector performance. To optimize detector performance, the total unintended acceptor concentration should be kept below about the mid 10.sup.12 /cm.sup.3 range. Thus, an effective method of measuring compensator levels soon after fabrication of semiconductor epitaxial layers has been sought. The measurement process should be accurate, reliable, and able to provide a compensator concentration profile within hours of epitaxial growth on test wafers.
The technique of cryogenic capacitance versus voltage (C-V) measurement has been used to determine acceptor concentration profiles in n-type silicon and to determine donor concentration profiles in p-type silicon. As used for profiling compensator concentrations, the cryogenic C-V method involves measurement of the space charge in the depletion region of an MIS (metal-insulator-semiconductor) type of capacitor at low temperatures. At cryogenic temperatures, below about 20 degrees Kelvin, the undoped semiconductor layer in an IBC device acts like an insulator. The space charge in the depletion region is due to ionized compensators. The depletion region thickness, determined from the capacitance (C) per unit area, is changed by varying the applied voltage (V). This is the blocked impurity band detector model known in the prior art. In the case of an n-type semiconductor with low compensation at low temperature, the carriers are D.sup.+ charges (i.e., electron vacancies on donors) in the impurity band. The resulting space charge in the region depleted of carriers is due to negatively charged acceptors.
As performed in the past, C-V methods required measurement of fully processed devices that took six weeks or more to fabricate. This limitation prevented the use of C-V techniques for fast characterization of epitaxial layers. As a result, these C-V methods could not be used for evaluating and selecting material for processing BIB detectors, SSPMs, or intrinsic event discriminators. Thus, there has been a need for a method of characterizing an epitaxial layer early in the process to improve and accelerate the fabrication of infrared detectors.